U.S. patent application number 14/159356 was filed with the patent office on 2014-05-15 for method and arrangement in a radio communications system for supporting dtx.
This patent application is currently assigned to Telefonaktiebolaget L M Ericsson (publ). The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Robert Baldemair, Erik Dahlman, Pal Frenger.
Application Number | 20140135022 14/159356 |
Document ID | / |
Family ID | 41510541 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140135022 |
Kind Code |
A1 |
Frenger; Pal ; et
al. |
May 15, 2014 |
Method and Arrangement in a Radio Communications System for
Supporting DTX
Abstract
A method in a first base station for supporting DTX is provided.
The first base station serves a first cell being in an active mode.
The first base station communicates with a user equipment within
the first cell. The first base station is comprised in a radio
communications system further comprising the user equipment and a
second base station serving a second cell being in a non observable
mode. The first base station sends to the second base station, a
request to switch the second cell state from a non observable mode
to an observable mode. It further sends to the user equipment or to
the second base station, a request to perform signalling between
the user equipment and the second base station for quality
measurements. The first base station then obtains information that
handover is feasible, based on quality measurement of the performed
signalling. The first base station sends to the second base
station, a request to prepare handover of the user equipment from
the first cell to the second cell, and further to the user
equipment, a command to perform handover to the second cell.
Inventors: |
Frenger; Pal; (Linkoping,
SE) ; Baldemair; Robert; (Solna, SE) ;
Dahlman; Erik; (Bromma, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stockholm |
|
SE |
|
|
Assignee: |
Telefonaktiebolaget L M Ericsson
(publ)
Stockholm
SE
|
Family ID: |
41510541 |
Appl. No.: |
14/159356 |
Filed: |
January 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13319258 |
Mar 9, 2012 |
8639252 |
|
|
PCT/SE2009/050503 |
May 8, 2009 |
|
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|
14159356 |
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Current U.S.
Class: |
455/437 |
Current CPC
Class: |
H04W 52/0216 20130101;
H04W 76/28 20180201; H04W 36/0058 20180801; H04W 36/0055 20130101;
H04W 92/20 20130101; Y02D 30/70 20200801 |
Class at
Publication: |
455/437 |
International
Class: |
H04W 36/00 20060101
H04W036/00; H04W 76/04 20060101 H04W076/04; H04W 52/02 20060101
H04W052/02 |
Claims
1. A method in a first base station for supporting Discontinuous
Transmission (DTX), the first base station serving a first cell,
the first cell being in an active mode, the first base station
communicating with a user equipment within the first cell over a
radio carrier, the first base station being comprised in a radio
communications system which radio communications system further
comprises the user equipment and a second base station serving a
second cell, the second cell being in a non observable mode for the
user equipment, the method comprising: sending to the second base
station, a request to switch the second cell state from non
observable mode to an observable mode for said user equipment;
sending to the user equipment or to the second base station, a
request to perform signalling between the user equipment and the
second base station for quality measurements; obtaining information
that handover is feasible, based on quality measurement of the
performed signalling; sending to the second base station, a request
to prepare handover of the user equipment from the first cell to
the second cell; and sending to the user equipment a command to
perform handover to the second cell.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/319,258, filed Mar. 9, 2012, which is the
National Stage of International Application No. PCT/SE2009/050503,
filed May 8, 2009, the disclosures of each of which are
incorporated herein by reference in their entirety.
TECHNICAL FIELD
[0002] The present invention relates to a method and an arrangement
in a first base station, a method and an arrangement in a second
base station and a method and an arrangement in a user equipment.
In particular, it relates to for supporting Discontinuous
Transmission (DTX) for power saving.
BACKGROUND
[0003] In a typical cellular system, also referred to as a radio
communications system, wireless terminals, also known as mobile
stations and/or User Equipment units (UEs) communicate via a Radio
Access Network (RAN) to one or more core networks. The wireless
terminals can be mobile stations or user equipment units such as
mobile telephones also known as "cellular" telephones, and laptops
with wireless capability, e.g., mobile termination, and thus can
be, for example, portable, pocket, hand-held, computer-included, or
car-mounted mobile devices which communicate voice and/or data with
radio access network.
[0004] The radio access network covers a geographical area which is
divided into cell areas, with each cell area being served by a base
station, e.g., a Radio Base Station (RBS), which in some networks
is also called "NodeB" or "B node" and which in this document also
is referred to as a base station. A cell is a geographical area
where radio coverage is provided by the radio base station
equipment at a base station site. Each cell is identified by an
identity within the local radio area, which is broadcast in the
cell. The base stations communicate over the air interface
operating on radio frequencies with the user equipment units within
range of the base stations.
[0005] In some versions of the radio access network, several base
stations are typically connected, e.g., by landlines or microwave,
to a Radio Network Controller (RNC). The radio network controller,
also sometimes termed a Base Station Controller (BSC), supervises
and coordinates various activities of the plural base stations
connected thereto. The radio network controllers are typically
connected to one or more core networks.
[0006] The Universal Mobile Telecommunications System (UMTS) is a
third generation mobile communication system, which evolved from
the Global System for Mobile Communications (GSM), and is intended
to provide improved mobile communication services based on Wideband
Code Division Multiple Access (WCDMA) access technology. UMTS
Terrestrial Radio Access Network (UTRAN) is essentially a radio
access network using wideband code division multiple access for
user equipment units (UEs). The Third Generation Partnership
Project (3GPP) has undertaken to evolve further the UTRAN and GSM
based radio access network technologies.
[0007] Being "green" has quickly gone from just a marketing hype to
a key requirement for customers. This is true today for almost all
sectors in society and the telecom market is no exception. The
world today is facing a global energy crisis as people start to
realize that our carbon based economy is no longer sustainable. Two
areas that will become significant parts of a future sustainable
economy are energy savings and telecommunications. Energy savings
are by far the quickest and cheapest way of reducing carbon dioxide
emissions to the atmosphere and telecommunication can significantly
reduce the need for physical transportation of people and services.
In 3rd Generation Partnership Project (3GPP) the work on the first
release of the 3G Long Term Evolution (LTE) system, denoted LTE
Rel-8 in 3GPP jargon, was finalized during 2008. The specifications
of the next releases (denoted LTE Rel-9 and LTE Rel-10) are
expected to be available 2009 and 2010 respectively. In LTE Rel-8
the maximum bandwidth is 20 MHz while in Rel-10 support for
aggregation of multiple component carriers resulting in a total
bandwidth of up to 100 MHz is expected to be specified, which LTE
Rel-8 system and later releases of the LTE system all consume
power.
SUMMARY
[0008] It is therefore an objective of the present solution to
provide a mechanism enabling power saving base station operation
based on DTX.
[0009] According to a first aspect of the present invention, the
object is achieved by a method in a first base station for
supporting Discontinuous Transmission (DTX). The first base station
serves a first cell. The first cell is in an active mode. The first
base station communicates with a user equipment within the first
cell over a radio carrier. The first base station is comprised in a
radio communications system. The radio communications system
further comprises the user equipment and a second base station
serving a second cell. The second cell state is in a non observable
mode. The first base station sends to the second base station, a
request to switch the second cell from a non observable mode to an
observable mode for said user equipment 120. The first base station
further sends to the equipment or to the second base station, a
request to perform signalling between the user equipment and the
second base station for quality measurements. The first base
station then obtains information that handover is feasible. The
information is based on quality measurement of the performed
signalling. Upon receiving this information, the first base station
sends to the second base station, a request to prepare handover of
the user equipment from the first cell to the second cell. The
first base station then sends to the user equipment, a command to
perform handover to the second cell.
[0010] According to a second aspect of the present invention, the
object is achieved by a method in a second base station for
supporting DTX. The second base station serves a second cell. The
second cell is in a non observable mode. The second base station is
comprised in a radio communications system. The radio
communications system further comprises a first base station
communicating with a user equipment over a radio carrier being
active. After receiving from the first base station, a request to
switch the second cell state from a non observable mode to an
observable mode for said user equipment 120, the second base
station switching the second cell state to an observable mode for
said user equipment 120. After receiving from the first base
station, a request to perform signalling between the user equipment
and the second base station for quality measurements, the second
base station signals to or from the user equipment. The second base
station receives from the first base station, a request to prepare
a handover of the user equipment from the first cell to the second
cell. The handover request is based on quality measurement
performed on said signalling. When the second cell state is in
observable mode the second base station switches the second cell
state to active mode, and prepares handover of the user equipment
from the first cell to the second cell. In some embodiments, base
station DTX is performed within the second base station, when the
second cell is in a non observable mode.
[0011] According to a third aspect of the present invention, the
object is achieved by a method in a user equipment for supporting
DTX. The user equipment is present in a first cell communicating
over a radio carrier with a first base station serving the first
cell. The first cell is set in an active mode. The user equipment
and the first base station are comprised in a radio communications
system. The radio communications system further comprises a second
base station serving a second cell. The second cell is set in a non
observable mode. The user equipment receives from the first base
station, a request to perform channel sounding signalling to be
observed by the second base station for quality measurements. The
second base station has been requested by the first base station to
switch the second cell state from a non observable mode to an
observable mode for said user equipment 120. The user equipment
then signals channel sounding to be observed by the second base
station for quality measurements. After receiving from the first
base station, a command to perform handover to the second cell, the
user equipment performs the commanded handover from the first cell
to the second cell. The handover command is based on quality
measurement performed by the second base station on the signalled
channel sounding observed by the second base station.
[0012] According to a fourth aspect of the present invention, the
object is achieved by an arrangement in a first base station for
supporting DTX. The first base station serves a first cell. The
first cell is adapted to be in an active mode. The first base
station is arranged to communicate with a user equipment within the
first cell over a radio carrier. The first base station is
comprised in a radio communications system. The radio
communications system further comprises the user equipment and a
second base station serving a second cell. The second cell is
adapted to be in an non observable mode. The first base station
arrangement comprises a sending unit configured to send to the
second base station, a request to switch the second cell state from
a non observable mode to an observable mode for said user equipment
120. The sending unit is further configured to send to the user
equipment or to the second base station, a request to perform
signalling between the user equipment and the second base station
for quality measurements. The first base station arrangement
further comprises an obtaining unit configured to obtain
information that handover is feasible, based on quality measurement
of the performed signalling. The sending unit is further configured
to send to the second base station, a request to prepare handover
of the user equipment from the first cell to the second cell. The
sending unit is further configured to send to the user equipment a
command to perform handover to the second cell.
[0013] According to a fifth aspect of the present invention, the
object is achieved by an arrangement in a second base station for
supporting DTX. The second base station serves a second cell. The
second cell is adapted to be in an non observable mode. The second
base station is comprised in a radio communications system. The
radio communications system further comprises a first base station
arranged to communicate with a user equipment over a radio carrier
being active. The second base station arrangement comprises a
receiving unit configured to receive from the first base station, a
request to switch the second cell state from a non observable mode
to an observable mode for said user equipment 120. The second base
station arrangement further comprises a switching unit configured
to switch the second cell state from a non observable mode to an
observable mode for said user equipment 120. The receiving unit is
further configured to receive from the first base station, a
request to perform signalling between the user equipment and the
second base station for quality measurements. The second base
station arrangement further comprises a signalling unit configured
to signal to or from the user equipment, which signal is to be used
for quality measurement. The receiving unit is further configured
to receive from the first base station, a request to prepare a
handover of the user equipment from a first cell served by the
first base station to a second cell served by the second base
station. The handover request is based on quality measurement on
said signalling. The switching unit is further configured to switch
the second cell state from observable mode to active mode, when the
second cell state is in observable mode. The second base station
arrangement further comprises a preparing unit configured to
prepare handover of the user equipment from the first cell to the
second cell.
[0014] According to a sixth aspect of the present invention, the
object is achieved by an arrangement in a user equipment for
supporting DTX. The user equipment is arranged to be in a first
cell and is adapted to communicate over a radio carrier with a
first base station serving the first cell. The first cell is
adapted to be in active mode. The user equipment and the first base
station are comprised in a radio communications system. The radio
communications system further comprises a second base station
adapted to serve a second cell, the second cell being arranged to
be in an non observable mode. The user equipment arrangement
comprises a signalling unit configured to receive from the first
base station, a request to perform channel sounding signalling to
be observed by the second base station for quality measurements.
The second base station has been requested by the first base
station to switch the second cell state from a non observable mode
to observable mode. The signalling unit is further configured to
signal channel sounding to be observed by the second base station
for quality measurements. The signalling unit is further configured
to receive from the first base station a command to perform
handover to the second cell. The handover command is based on
quality measurement performed by the second base station on the
signalled channel sounding observed by the second base station. The
user equipment arrangement further comprises a performing unit
configured to perform a handover from the first cell to the second
cell.
[0015] Since the first base station requests the second base
station switch the second cell state from a non observable mode to
an observable mode for said user equipment 120, and a requests the
second base station or the user equipment to perform signalling
between the user equipment and the second base station for quality
measurements, handover to the second cell can be performed in spite
of the second base station initially being in non observable mode,
in which non observable mode power saving base station operation
based on DTX is enabled.
[0016] An advantage with the present solution is that a network
element can remain in low power consuming mode for a longer time.
Without then present solution, a base station in DTX will have to
periodically or pseudo randomly leave DTX mode to enable non-served
user equipments to measure.
[0017] A further advantage with the present solution is that the
time to switch up from a DTX mode to an active mode will be
significantly shorter with the present solution. Since, with the
present solution, the mode switch of the second base station is
event triggered there is no need to wait for a periodic or
pseudorandom timer to expire before entering an observable mode.
Instead the second base station can switch to an observable mode
for said user equipment 120 immediately after a request is received
from the first base station. To enable a fast switching time with a
state of the art timer based solution periodic or pseudorandom the
DTX time would need to be reduced significantly and that would
limit the potential energy savings.
[0018] Also with the current invention it can be avoided that the
second cell unnecessarily enters an observable mode, i.e. non DTX
mode, or an observing mode, i.e. non DRX mode. Each time the second
cell becomes observable or starts to observe cost must paid in
terms of increased energy consumption. When a handover measurement
is required then that cost is well motivated, but if the second
base station would enter an observable mode, i.e. non DTX mode, or
an observing mode, i.e. non DRX mode periodically as in state of
the art solutions then it will often be wasted energy by
transmitting signals from the second base station that no user
terminal is measuring on or by performing measurements in the
second base station even though no user terminal is transmitting
anything for the second base station to measure on.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The invention is described in more detail with reference to
attached drawings illustrating exemplary embodiments of the
invention and in which:
[0020] FIG. 1 is a schematic block diagram illustrating a radio
frame according to prior art.
[0021] FIG. 2 is a schematic block diagram illustrating embodiments
of a radio communications system.
[0022] FIG. 3 is a schematic block diagram illustrating embodiments
of a radio communications system.
[0023] FIG. 4 is a combined schematic block diagram and flowchart
depicting embodiments of a method.
[0024] FIG. 5 is a combined schematic block diagram and flowchart
depicting embodiments of a method.
[0025] FIG. 6 is a flowchart depicting embodiments of a method in a
first base station.
[0026] FIG. 7 is a schematic block diagram illustrating embodiments
of a first base station arrangement.
[0027] FIG. 8 is a flowchart depicting embodiments of a method in a
second base station.
[0028] FIG. 9 is a schematic block diagram illustrating embodiments
of a second base station arrangement.
[0029] FIG. 10 is a flowchart depicting embodiments of a method in
a user equipment.
[0030] FIG. 11 is a schematic block diagram illustrating
embodiments of a user equipment arrangement.
DETAILED DESCRIPTION
[0031] As part of the present solution a problem will firstly be
defined and discussed. To introduce energy saving features,
increased support for eNB Discontinuous Transmission (DTX) is
required in LTE Rel-10. Several different types of downlink DTX
could be considered. In the context of LTE 3 types of Down Link
(DL) DTX modes could be defined: Short DTX could be defined as DTX
that is fully backwards compatible with LTE Rel-8. Basically short
DL DTX is then limited to one or a few OFDM symbols in which no
cell specific reference symbols need to be transmitted. Medium DTX
could be defined as DTX that is non backwards compatible with
Rel-8, e.g. a DTX duration longer than one sub-frame (1 ms) but
shorter than a radio frame (10 ms). Finally long DTX could be
defined as a DTX duration that makes a carrier invisible also for
Rel-10 UEs e.g. a DTX duration equal to one or several radio
frames. The long DL DTX might also be denoted eNB sleep.
[0032] At a first glance it seems rather straightforward to
introduce support for medium or long eNB DTX in LTE Rel-10. FIG. 1
shows a LTE radio frame with 72 center sub carriers, introducing
eNB DTX for four ms in subframe no. 1, 2, 3, 4, 6, 7, 8 and 9. For
example, the LTE Rel-10 specification may be slightly changed so
that when there is little or no user plane data traffic, the eNB
does not have to transmit Cell Specific Reference Symbols (CSRS) in
every subframe. In this example, CSRS may only be mandatory in
sub-frames 0 and 5 where also Primary Synchronization Signals (PSS)
and Secondary Synchronization Signals (SSS) and Broadcast CHannel
(BCH) are transmitted. CSRS also needs to be transmitted in those
sub-frames that carry System Information Blocks, (SIBs). SIB1 is
transmitted in the fifth sub-frame of every radio frame, SIBx where
x>2 is configurable with very low duty cycle. In order for this
to be allowed the UE behavior that the standard specifies may for
example be changed so that UEs are only allowed to perform mobility
measurements during sub-frames 0 and 5.
[0033] The fact that the CSRS are not transmitted in every
sub-frame may also likely affect the Channel Quality Indicator
(CQI) measurements. However already in Rel-8 it is possible to
specify when in time the CQI measurements are to be performed. In
Rel-8 the CQI measurements are performed 4 sub-frames before the UE
are scheduled to report the CQI. No time domain filtering of CQI
estimates are performed in the UE. It might be necessary to
reconsider if this mechanism is sufficient also for Rel-10 or if
some more flexibility is required.
[0034] In case UEs measure the CQI in sub-frames other than 0 and 5
then they can not assume that there is any frequency correlation
since the eNB may not always transmit CSRS in all resource blocks.
Alternatively the UEs could detect an "all zero CQI" measurement as
an indication that the eNB does not need any CQI report from the
UE.
[0035] UE channel estimation is also affected. A slight degradation
in channel estimation accuracy is expected since UEs can not
utilize time and frequency correlation between resource blocks
(unless they are adjacent to subframe 0 or 5). This however is
already the case for TDD, where one cannot do interpolation between
all subframes as some subframes are UL subframes. Hence this is not
a fundamental problem.
[0036] There are many alternatives to the solution outlined in FIG.
1. UE mobility measurements can be limited to [0037] the centre 6
resource blocks; and/or [0038] a single antenna port (e.g. antenna
port 0) and/or [0039] the PSS and SSS signals only; and/or [0040]
subframe 0 only, i.e. not both sub-frame 0 and 5 as in FIG. 1.
[0041] It is also possible that a future non legacy LTE release
(e.g. Rel-10) defines a new set of reference symbols for non
backwards compatible extension carriers. Current discussions in
3GPP mention two new sets of reference symbols: demodulation
reference symbols (DM-RS) and channel state information reference
symbols (CSI-RS). If new reference symbols are defined (e.g. a new
set of mobility measurement reference signals of the DM-RS or CSI
RS mentioned above) then it is likely that UE mobility measurements
are defined on a sub-set of the new reference symbols.
[0042] To enable DTX periods longer than 4 ms one could imagine
that an eNB sleep mode is also defined for LTE Rel-9 or LTE Rel-10.
Periodically a sleeping eNB could transmit all signals needed for
UEs to measure and attach to the cell, i.e. PSS, SSS, BCH, SIB1,
SIB2, CSRS during a short active period duration such as e.g. 50
ms. The active period is then followed by a much longer inactive
period such as e.g. 450 ms where nothing is transmitted from the
eNB. The active period can be compatible with LTE Rel-8 or a later
release, e.g. LTE Rel-10.
[0043] The problem with the energy saving solutions discussed above
is that a user equipment can not access a cell being in DTX mode
since a cell being in a DTX mode is invisible to this user
equipment, and the serving base station will not receive any
handover measurements from the user equipment that can trigger a
handover.
[0044] It is therefore a further objective of the present solution
to provide the means required to making it possible for a user
equipment to access a cell being in a non observable cell state and
therefore is not transmitting anything, or access a cell in a DTX
mode not supported by the user equipment.
[0045] FIG. 2 depicts a radio communications system 100, such as
e.g. the E-UTRAN, also known as LTE, LTE-Adv, 3rd Generation
Partnership Project (3GPP) WCDMA system, Global System for Mobile
communications/Enhanced Data rate for GSM Evolution (GSM/EDGE),
Wideband Code Division Multiple Access (WCDMA), Worldwide
Interoperability for Microwave Access (WiMax), or Ultra Mobile
Broadband (UMB). The radio communications system 100 supports
different releases of a standardized radio specification such as a
standardized LTE specification or a standardized High Speed Packet
Access (HSPA) specification.
[0046] The radio communications system 100 comprises a first base
station 110 serving a first cell 115. The first base station 110
may be a base station such as a NodeB, an eNodeB or any other
network unit capable to communicate with a user equipment being
present in the first cell over a radio carrier. The first base
station 110 communicates with a user equipment 120 being present
within the first cell 115 over a radio carrier 125. The first cell
115 is in an active mode, this means that the radio carrier 125 in
the first cell 115 is up and running e.g. with a DTX mode supported
by the user equipment 120. With active mode in this context is
meant that the transmission mode of the serving cell, i.e. the
first cell 115 is such that data communication is possible between
the user equipment 120 and the first cell 115. Thus, as seen from
the perspective of the user equipment 120, only the serving cell,
i.e. the first cell 115 can be in active mode and a non serving
cell, i.e. the second cell 135 can not. However a non serving cell,
i.e. the second cell 135 can be in a DTX mode that is either
observable or non observable for the user equipment. In the example
of FIG. 2 and only for illustration, the first base station 110
further communicates with other user equipments 127 in the first
cell 115 over other radio carriers 129. The user equipment 120 may
be a mobile phone, a Personal Digital Assistant (PDA), or any other
network unit capable to communicate with a base station over a
radio channel.
[0047] The radio communications system 100 further comprises a
second base station 130 serving a second cell 135. The second base
station 130 may be a base station such as a NodeB, an eNodeB or any
other network unit capable to communicate with a user equipment
present in the second cell 135, over a radio carrier when the cell
state is in active mode. However, in FIG. 2 the second cell is not
in an active mode but in a non observable mode which means that the
carrier within the second cell 135, seen from the user equipment
120 perspective, is not transmitting anything or it is in a DTX
mode not supported by the user equipment 120. Note that it is
possible to consider the case that a new DTX mode is introduced in
a new release of a standardized system specification, such as 3GPP
LTE. In that case only user equipments that comply with the new
standard release will be able to observe a cell that is operating
using the new DTX mode. Thus a DTX mode that is observable for one
user equipment may be non observable for another user
equipment.
[0048] The general idea of the present solution will now be
described. In an exemplary scenario referred to in FIG. 2, the user
equipment 120 would be better served by second base station 130 and
the second cell 135. The second cell 135 being in a non observable
mode is not visible for the user equipment 120.
[0049] To start a hand over procedure to hand over the user
equipment 120 to the second cell 135, the first base station 110
requests the second base station 130 to switch the second cell
state from a non observable mode to an observable mode for said
user equipment 120. An observable mode means that from the
perspective of the user equipment 120, the carrier in the second
cell 135 is temporarily transmitting with a DTX mode supported by
the user equipment 120 for the sole purpose of allowing mobility
measurements. But neither the user equipment 120 nor any other user
equipments present within the second cell 135 are actively
receiving user plane data on the carrier.
[0050] The second base station 130 switches the second cell state
to an observable mode for said user equipment 120. This switch
makes the second cell 135 visible to the user equipment 120.
[0051] The first base station 110 then requests the user equipment
120 or the second base station 130 to perform signalling between
the user equipment 120 and the second base station 130 for quality
measurements.
[0052] The first base station 110 obtains information that handover
is feasible, based on quality measurement of the performed
signalling.
[0053] The first base station 110 then requests the second base
station 130 to prepare handover of the user equipment 120 to the
second cell 135, and commands the user equipment 120 to perform
handover to the second cell 135.
[0054] The handover to the second cell is performed by the user
equipment 110 ending up in the scenario illustrated in FIG. 3. The
second base station 130 switches to active mode. The user equipment
110 and the second base station 130 may then start communicating
with each other over a radio carrier.
[0055] As a consequence of the present solution in methods
described above and below, a cell may request that a non observable
neighboring cell becomes observable.
[0056] So e.g. when load is high a base station serving a cell may
check if any neighboring cells can take over some of the
traffic.
[0057] In the present solution, the mode terminology is defined
from the user equipment 120 point of view. Therefore it is only the
serving cell, i.e. the first cell 115 that can be in an "active"
mode. A non serving cell, i.e. the second cell 135, can only be
"observable" or "non observable" as far as this user equipment 120
is concerned. The second cell 135 may also be "observing" but it
can not be "active", at least not until the user equipment 120 has
performed a handover to the second cell 130 after which the second
cell 135 is no longer referred to as the second cell 135.
Furthermore, note that the term "observable" denotes that the
characteristics of the radio channel between the user equipment 120
and the second base station 130 may be determined. Thus the term
"observable" is used to describe both the case that the second cell
135 transmits a reference signal on which said user equipment 120
can perform mobility measurements; as well as the case that the
second cell 135 is prepared for performing measurements needed for
mobility decisions on a reference signal transmitted by said user
equipment 120.
[0058] The user equipment 120 that is served by the first cell 115,
i.e. the first cell 115 is thus in "active" mode, does not care if
the second cell 135 happens to actively serve some other user
equipments 127 or not. That the second cell 135 is in active mode
is irrelevant unless we assume that any cell that is in active mode
is automatically also observable for all user equipments in
neighboring cells. That might not be the case since the second cell
135 might serve a LTE Rel-10 user equipment using a Rel-10
transmission format, hence it is in active mode as seen from this
user equipment, and the user equipment in the neighboring cell
might be a Rel-8 user equipment that is not capable of measuring on
this format, hence the second cell is non observable as seen from
this user equipment. So when reading "observable" and "non
observable" it is from the viewpoint of the user equipment 120.
What is observable for one user equipment (e.g. a Rel-10 user
equipment) can be non observable for another user equipment (e.g. a
Rel-8 user equipment). The term "observable" also refers to
measuring on a specific signal that the second cell knows is being
transmitted from the user equipment 120 served by the first cell
115.
[0059] Thus, "active mode" is used to describe the mode of the cell
currently serving the user equipment, "non observable" or
"observable" mode is used to describe if the characteristics of the
radio channel between the user equipment 120 and the second base
station 130 can be determined or not.
[0060] In case the second cell 135 is "non observable" the it can
either be requested to become "observable" i.e. start to send
reference signals according to a format that the user equipment 120
can measure on; or be prepared to measure on channel sounding
transmissions from the user equipment 120. It does not matter
whether the second cell 135 is "active" or not until the user
equipment 120 enters the second cell 135.
[0061] Note that for the sake of simplifying the description of
this invention, only the case when each cell transmits a single
carrier described. In case several carriers are transmitted from a
base station then it is possible to view that arrangement as
several cells with one carrier allocated to each cell.
Alternatively one the multicarrier arrangement can be viewed as a
single cell with several carriers that are allocated to it. In the
present solution, it is chosen to adopt the commonly used
nomenclature that a cell transmit only one carrier and in case
having multiple carrier arrangements then the additional carriers
are viewed as additional cells.
[0062] Currently carrier aggregation is being defined by 3GPP as a
component technology for LTE Rel-10. The idea is that a Rel-10 user
equipment shall be able to aggregate several component carriers
transmitted from the same base station. In that case it is not
decided yet in 3GPP if a Rel-10 user equipment will view the
component carriers as one cell or several cells. In case the
component carriers are visible to a Rel-8 user equipments they will
however be viewed as different cells by the Rel-8 user equipments.
Therefore, the present solution also covers a multicarrier
arrangement where a cell has several carriers assigned to it.
[0063] Cell state changes may be exchanged for example directly
between the first base station 110 and the second base station 130
e.g. over X2/S1 or over an O&M interface e.g. according to a
Listener/Reporter pattern. The X2 interface is, as defined in 3GPP,
a direct logical interface between two eNodeBs; the S1 interface is
the interface between the eNodeB and the mobility management entity
(MME); the O&M interface is the interface between the eNodeB
and the operation and support system (OSS). Thus a decision to
enter a non observable mode may be taken locally in the base
station but when the non observable mode of a carrier is changed to
an observable mode for said user equipment 120, all neighboring
cells, i.e. all listeners may be informed by this transmission by
the first cell 135, i.e. the reporter. Also component carrier state
information such as e.g. legacy/short DTX/long DTX may be exchanged
between the first base station 110 and the second base station 130
and other base stations in case there are several component
carriers in the first base station 110 and/or second base station
130.
[0064] Some embodiments of the present solution will now be
described.
[0065] A method according to some first embodiments is depicted in
the combined flowchart and signaling diagram in FIG. 4. The user
equipment 120 may probably be better served by the second base
station 130 and the second cell 135. The second cell 135 being in a
non observable mode is not visible for the user equipment 120.
[0066] The method steps below must not be taken in the order
described below, but may be taken in any suitable order.
[0067] Step 401
[0068] The first base station 110 requests the second base station
130 to switch to observable mode to be able to start sending
reference signals. This may be performed at the same time as the
next step, or implicitly be performed by the next step. This step
may e.g. be triggered by information received that the user
equipment 120 requires a service that not is provided by the first
base station 110. This step may also e.g. be triggered by high
traffic load, such as the traffic load exceeding a predetermined
threshold value.
[0069] Step 402
[0070] The second base station switches to an observable mode for
said user equipment 120.
[0071] Step 403
[0072] The first base station 110 further requests the second base
station 130 to perform signaling between the user equipment 120 and
the second base station 130 for quality measurements. In these
first embodiments this is performed by requesting the second base
station 110 to start transmitting reference signals such as e.g.
pilot signals.
[0073] Step 404
[0074] The second base station 130 sends reference signals.
[0075] Step 405
[0076] The user equipment 110 listens to the reference signals sent
by the second base station 130 and performs quality measurements on
said signals. These measurements are in LTE denoted reference
signal received power (RSRP) measurements and they are used to
perform handover decisions.
[0077] Step 406
[0078] When the user equipment 110 decides that handover to the
second cell is feasible based on said measurements in step 404, it
sends a hand over request to the first base station 110.
[0079] Step 407
[0080] The first base station 110 then requests the second base
station 130 to prepare handover of the user equipment 120 to the
second cell.
[0081] Step 408
[0082] The first base station 110 then commands the user equipment
120 to perform handover to the second cell 135.
[0083] Step 409
[0084] The user equipment 120 performs handover to the second cell
135 and may then start to communicate with the second base station
130.
[0085] A method according to some second embodiments is depicted in
the combined flowchart and signaling diagram in FIG. 5. Also in
these embodiments, the user equipment 120 may be better served by
the second base station 130 and the second cell 135. The second
cell 135 being in a non observable mode is not visible for the user
equipment 120.
[0086] The method steps below must not be taken in the order
described below, but may be taken in any suitable order.
[0087] Step 501
[0088] The first base station 110 requests the second base station
130 to switch to observable mode to be able to start sending
reference signals. In these second embodiments this may be
performed by requesting the second base station 110 to start
measuring on channel sounding transmission e.g. on a particular
channel sounding which is signalled by the user equipment 120. This
may be performed at the same time as the next step, or implicitly
be performed by the next step. This step may e.g. be triggered by
information received that the user equipment 120 requires a service
that not is provided by the first base station 110. This step may
also e.g. be triggered by high traffic load, such as the traffic
load exceeding a predetermined threshold value.
[0089] Channel Sounding Reference Signal (SRS) transmission are
defined in LTE for the purpose of probing the uplink radio channel
from the user equipment to the base station. The purpose of
introducing SRS in LTE is to enable channel dependent scheduling
also in the uplink.
[0090] Step 502
[0091] The second base station 130 switches to an observable mode
for said user equipment 120, that in this case represented by an
observing mode.
[0092] Step 503
[0093] The first base station 110 further requests the user
equipment 120 to perform signaling between the user equipment 120
and the second base station 130 for quality measurements. In these
embodiments this may be performed by commanding the user equipment
120 to transmit channel sounding signals.
[0094] Step 504
[0095] The user equipment 120 transmits channel sounding
signals.
[0096] Step 505
[0097] The second base station 130 listens to the channel sounding
sent by the user equipment 120 and performs quality measurements on
said channel sounding signals. The received power on the channel
sounding reference signal transmission from the user equipment 120
may be measured and the resulting measurement value is used by the
radio network for evaluating if the user equipment 120 shall
perform a handover to the second base station 130 or not.
[0098] Step 506
[0099] The second base station 130 sends measurement reports to the
first base station 110, regarding the results of the quality
measurements on said channel sounding signals.
[0100] Step 507
[0101] The first base station 110 reads the measurement reports
from the second base station 130 and decides when it is feasible to
perform handover based on the measurement report, i.e. information
is obtained that it is feasible to perform handover.
[0102] Step 508
[0103] When decided that it is feasible to perform handover of the
user equipment 120 to the second cell 135, the first base station
110 requests the second base station 130 to prepare handover of the
user equipment 120 to the second cell.
[0104] Step 509
[0105] The first base station 110 then commands the user equipment
120 to perform handover to the second cell 135.
[0106] Step 510
[0107] The user equipment 120 performs handover to the second cell
135 and may then start to communicate with the second base station
130.
[0108] The method steps in the first base station 110 for
supporting DTX, according to some embodiments will now be described
with reference to a flowchart depicted in FIG. 6. As mentioned
above the first base station 110 serves the first cell 115. The
first cell 115 is in active mode. The first base station 110
communicates with the user equipment 120 within the first cell 115
over a radio carrier. The first base station 110 is comprised in a
radio communications system, which radio communications system
further comprises the user equipment 120 and the second base
station 130 serving the second cell 135. The second cell state is
in a non observable mode.
[0109] The method comprising the following steps, which steps may
as well be carried out in another suitable order than described
below:
[0110] Step 601
[0111] This is an optional step. The first base station 110 may in
some embodiments detect or receive information from the user
equipment 120, that the user equipment 120 requires a service that
not is provided by the first base station 110. It may e.g. include
subscription information such as e.g. that only high paying user
equipments can wake up a micro cell.
[0112] Step 602
[0113] This step is the first step, if optional step 601 is not
performed. The first base station 110 sends to the second base
station 130, a request to switch the second cell state from a non
observable mode to an observable mode for said user equipment
120.
[0114] In some embodiments, this step is triggered when the traffic
load within the first cell exceeds a predetermined threshold
value.
[0115] In some embodiments, wherein the optional step 601 is
performed, this step is triggered upon receiving the information
that the user equipment 120 requires a service that not is provided
by the first base station 110.
[0116] Step 603
[0117] In this step the second base station 130 sends to the user
equipment 120 or to the second base station, a request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements.
[0118] In some first embodiments, the request to perform signalling
between the user equipment 120 and the second base station 130 for
quality measurements, is sent to the second base station 130, and
is represented by a request to send reference signals to be
measured on by the user equipment 120.
[0119] In some second embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements is represented by a request to
the user equipment 120 to transmit channel sounding signals, and a
request to the second base station 130 to measure on said channel
sounding signal transmission.
[0120] Step 604
[0121] The second base station then obtains information that
handover is feasible, based on quality measurement of the performed
signalling.
[0122] The obtained information that handover is feasible based on
quality measurement of the performed signalling, may in the first
embodiments be represented by a handover request from the user
equipment 120 based on quality measurement of the reference signal
by the user equipment 120.
[0123] In the other embodiments, the obtained information that
handover is feasible based on quality measurement of the performed
signalling, may be represented by receiving from the second base
station 130 a measurement report based on quality measurement
performed by the second base station 130 on the channel sounding
signals.
[0124] Step 605
[0125] After receiving the information, the first base station 110
sends to the second base station 130, a request to prepare handover
of the user equipment 120 from the first cell 115 to the second
cell 135.
[0126] Step 606
[0127] The first base station 110 also sends to the user equipment
120, a command to perform handover to the second cell 135. The
present method may end when this step and step 605 are
performed.
[0128] Step 607
[0129] This is an optional step. In this step the first base
station 110 may send to the second base station 130, information
about the current first cell state, and/or receive from the second
base station 110, information about the current second cell
state.
[0130] To perform the method steps above for supporting DTX, the
first base station 110 comprises an arrangement 700 depicted in
FIG. 7. As mentioned above, the first base station 110 serves the
first cell 115. The first cell state is adapted to be in active
mode. The first base station 110 is arranged to communicate with
the user equipment 120 within the first cell 115 over a radio
carrier. The first base station 110 is comprised in the radio
communications system 100. The radio communications system 100
further comprises the user equipment 120 and the second base
station 130 serving the second cell 135. The state of the second
cell 135 is adapted to be in a non observable mode for the user
equipment 120.
[0131] The first base station arrangement 700 comprising a sending
unit 710 configured to send to the second base station 130, a
request to switch the second cell state from a non observable mode
to an observable mode for said user equipment 120.
[0132] The sending unit 710 is further configured to send to the
user equipment 120 or to the second base station 130, a request to
perform signalling between the user equipment 120 and the second
base station 130 for quality measurements.
[0133] According to some first embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements is represented by a request to
the second base station 130 to send reference signals to be
measured on by the user equipment 120.
[0134] According to some second embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements is represented by a request to
the user equipment 120 to transmit channel sounding signals, and a
request to the second base station 130 to measure on said channel
sounding signal transmission.
[0135] The sending unit 710 is further configured to send to the
second base station 130, a request to prepare handover of the user
equipment 120 from the first cell 115 to the second cell 135.
[0136] The sending unit 710 is further configured to send to the
user equipment 120 a command to perform handover to the second cell
135.
[0137] In some embodiments, the sending unit 710 may further be
configured to send to the second base station 130 information about
the current first cell state.
[0138] The sending unit 710 may further be configured to be
triggered to send to the second base station 130, the request to
switch the second cell state from a non observable mode to
observable mode, when the traffic load within the first cell
exceeding a predetermined threshold value.
[0139] The first base station arrangement 700 further comprises an
obtaining unit 720 configured to obtain information that handover
is feasible, based on quality measurement of the performed
signalling.
[0140] According to the first embodiments, the obtained information
that handover is feasible based on quality measurement of the
performed signalling, may be represented by a handover request from
the user equipment 120 based on quality measurement of the
reference signal by the user equipment 120
[0141] According to the second embodiments, the obtain of
information that handover is feasible based on quality measurement
of the performed signalling, is represented by a measurement
report, based on quality measurement performed by the second base
station 130 on the channel sounding signals.
[0142] The first base station arrangement 700 may further comprise
a receiving unit 730 configured to receive from the second base
station 130 information about the current second cell state.
[0143] In some embodiments, the receiving unit 730 further is
configured to receive from the user equipment 120, information that
the user equipment 120 requires a service that not is provided by
the first base station 110. In these embodiments, the sending unit
710 may further be configured to be triggered to send to the second
base station 130, the request to switch the second cell state from
a non observable mode to observable mode, said information.
[0144] The method steps in the second base station 130 for
supporting DTX according to some embodiments will now be described
with reference to a flowchart depicted in FIG. 8. As mentioned
above, the second base station 130 serves the second cell 135. The
cell second cell 135 is in a non observable mode. The second base
station may perform DTX, when the second cell state is in non
observable mode. The second base station 130 is comprised in the
radio communications system 100. The radio communications system
100 further comprises the first base station 110 communicating with
the user equipment 120 over a radio carrier being active.
[0145] The method comprises the following steps that may as well be
carried out in another suitable order than described below:
[0146] Step 801
[0147] The second base station 130 receives from the first base
station 110, a request to switch the second cell state from a non
observable mode to an observable mode for said user equipment
120.
[0148] Step 802
[0149] The second base station 130 switches the second cell state
from a non observable mode to an observable mode for said user
equipment 120.
[0150] Step 803
[0151] The second base station 130 receives from the first base
station 110, a request to perform signalling between the user
equipment 120 and the second base station 130 for quality
measurements.
[0152] In some first embodiments the request to perform signalling
between the user equipment 120 and the second base station 130 for
quality measurements, is represented by a request to send reference
signals to be measured on by the user equipment 120.
[0153] In some second embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements is represented by a request to
the second base station 130 to measure on said channel sounding
signal transmission.
[0154] Step 804
[0155] The second base station 130 signals with, i.e. to or from
the user equipment 120. The signal is to be used by the user
equipment 120 for quality measurement.
[0156] In the first embodiments, the signalling to or from the user
equipment 120, may be represented by sending the requested
reference signals.
[0157] Step 805
[0158] This is an optional step relating to the second embodiments.
The second base station may measure the quality on channel sounding
signals observed from the user equipment 120.
[0159] Step 806
[0160] This is an optional step relating to the second embodiments.
The second base station 130 sends to the first base station 110, a
measurement report based on the quality measurement performed on
the channel sounding signals.
[0161] Step 807
[0162] The second base station 130 receives from the first base
station 110, a request to prepare a handover of the user equipment
120 from a first cell 115 served by the first base station 110 to
the second cell 135 served by the second base station 130. The
handover request is based on quality measurement on said
signalling.
[0163] Step 808
[0164] When the second cell state is in observable mode, the second
base station 130 switches the second cell state from observable
mode to active mode.
[0165] Step 809
[0166] As requested, the second base station prepares handover of
the user equipment 120 from the first cell 115 to the second cell
135. The present method may end at this step.
[0167] Step 810
[0168] This is an optional step. The second base station 130 may
send to the first base station 110 information about the current
second cell state. The present method may end at this step.
[0169] Step 811
[0170] This is also an optional step. The second base station 130
may receive from the first base station 110, information about the
current first cell state. The present method may end at this
step.
[0171] To perform the method steps above for supporting DTX, the
second base station 130 comprises an arrangement 900 depicted in
FIG. 9. As mentioned above, the second base station 130 serves the
second cell 135. The second cell 135 is adapted to be in a non
observable mode for the user equipment 120. Base station DTX may be
arranged to be performed within the second base station 130, when
the second cell state is in a non observable mode. The second base
station 130 is comprised in a radio communications system 100. The
radio communications system 100 further comprises the first base
station 110 arranged to communicate with the user equipment 120
over a radio carrier being active.
[0172] The second base station arrangement 900 comprises a
receiving unit 910 configured to receive from the first base
station 110, a request to switch the second cell state from a non
observable mode to an observable mode for said user equipment
120.
[0173] The receiving unit 910 is further configured to receive from
the first base station 110, a request to perform signalling between
the user equipment 120 and the second base station 130 for quality
measurements.
[0174] According to some first embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements may be represented by a
request to send reference signals to be measured on by the user
equipment 120.
[0175] According to some second embodiments, the request to perform
signalling between the user equipment 120 and the second base
station 130 for quality measurements, may be represented by a
request to the second base station 130 to measure on said channel
sounding signal transmission.
[0176] The receiving unit 910 is further configured to receive from
the first base station 110, a request to prepare a handover of the
user equipment 120 from a first cell 115 served by the first base
station 110 to a second cell 135 served by the second base station
130. The handover request is based on quality measurement on said
signalling.
[0177] The second base station arrangement 900 further comprising a
switching unit 920 configured to switch the second cell state from
a non observable mode to an observable mode for said user equipment
120.
[0178] The switching unit 920 is further configured to switch the
second cell state from observable mode to active mode, when the
second cell state is in observable mode.
[0179] The second base station arrangement 900 further comprises a
signalling unit 930 configured to signal to or from the user
equipment 120, which signal is to be used for quality
measurement.
[0180] According to the second embodiments, the signalling to or
from the user equipment 120, may be represented by sending the
requested reference signals.
[0181] The signalling unit 930 may further be configured to send to
the first base station 110, information about the current second
cell state.
[0182] The signalling unit 930 may further be configured to receive
from the first base station 110, information about the current
first cell state.
[0183] The second base station arrangement 900 further comprises a
preparing unit 940 configured to prepare handover of the user
equipment 120 from the first cell 115 to the second cell 135.
[0184] According to some of the second embodiments, the second base
station arrangement 900 further comprises a measuring unit 950
configured to measure the quality on channel sounding signals
observed from the user equipment 120.
[0185] In these embodiments, the signalling unit 930 may further be
configured to send to the first base station 110, a measurement
report based on the quality measurement performed on the channel
sounding signals.
[0186] The method steps in the user equipment 120 supporting DTX
according to some embodiments, will now be described with reference
to a flowchart depicted in FIG. 10. As mentioned above, the user
equipment 120 is present in the first cell 115 and communicates
over a radio carrier with the first base station 110. The first
base station 110 serves the first cell 115. The first cell 115 is
in an active mode. The user equipment 120 and the first base
station 110 are comprised in a radio communications system 100. The
radio communications system 100 further comprises the second base
station 130 serving the second cell 135. The second cell 135 is in
a non observable mode. The method comprises the following steps
that may as well be carried out in another suitable order than
described below:
[0187] Step 1001
[0188] This step is optional. According to a first embodiment, the
user equipment 120 may send to the first base station 110,
information that the user equipment 120 requires a service that not
is provided by the first base station 110.
[0189] Step 1002
[0190] If optional step 1001 is not performed, this step starts the
present method. The user equipment receives from the first base
station 110, a request to perform channel sounding signalling to be
observed by the second base station 130 for quality measurements.
The second base station 130 has been requested by the first base
station 110 to switch the second cell state from a non observable
mode to observable mode.
[0191] According to the first embodiment, this step may be a
response to the reception of the information received in step
1001.
[0192] Step 1003
[0193] The user equipment 120 signals channel sounding to be
observed by the second base station 130 for quality
measurements.
[0194] Step 1004
[0195] The user equipment 120 receives 1004 from the first base
station 110 a command to perform handover to the second cell 135,
which handover command is based on quality measurement performed by
the second base station 130 on the signalled channel sounding
observed by the second base station 130.
[0196] Step 1005
[0197] The user equipment 120 performs the commanded handover from
the first cell 115 to the second cell 135.
[0198] To perform the method steps above for supporting DTX, the
user equipment 120 comprises an arrangement 1100 depicted in FIG.
11. As mentioned above. The user equipment 120 is arranged to be in
the first cell 115. The user equipment is adapted to communicate
over a radio carrier with a first base station 110 serving the
first cell 115. The first cell 115 is adapted to be in active mode.
The user equipment 120 and the first base station 110 are comprised
in a radio communications system 100. The radio communications
system 100 further comprises the second base station 130 adapted to
serve the second cell 135. The second cell 135 is arranged to be in
a non observable mode for the user equipment 120.
[0199] The user equipment arrangement 1100 comprises a signalling
unit 1110 configured to receive from the first base station 110, a
request to perform channel sounding signalling to be observed by
the second base station 130 for quality measurements. The second
base station 130 has been requested by the first base station 110
to switch the second cell state from a non observable mode to
observable mode.
[0200] The signalling unit 1110 is further configured to signal
channel sounding to be observed by the second base station 130 for
quality measurements.
[0201] The signalling unit 1110 is further configured to receive
from the first base station 110 a command to perform handover to
the second cell 135. The handover command is based on quality
measurement performed by the second base station 130 on the
signalled channel sounding observed by the second base station
130.
[0202] In one embodiment, the signalling unit 1110 may further be
configured to send to the first base station 110, information that
the user equipment 120 requires a service that not is provided by
the first base station 110.
[0203] The user equipment arrangement 1100 further comprises a
performing unit 1120 configured to perform a handover from the
first cell 115 to the second cell 135.
[0204] The present mechanism for supporting DTX, may be implemented
through one or more processors, such as a processor 740 in the
first base station arrangement 700 depicted in FIG. 7, a processor
960 in the second base station arrangement 900 depicted in FIG. 9,
or a processor 1130 in the user equipment arrangement 1100 depicted
in FIG. 11, together with computer program code for performing the
functions of the present solution. The program code mentioned above
may also be provided as a computer program product, for instance in
the form of a data carrier carrying computer program code for
performing the present solution when being loaded into the first
base station 110, the second base station 130 or the user equipment
120. One such carrier may be in the form of a CD ROM disc. It is
however feasible with other data carriers such as a memory stick.
The computer program code can furthermore be provided as pure
program code on a server and downloaded to the first base station
110, the second base station 130 or the user equipment 120
remotely.
[0205] When using the word "comprise" or "comprising" it shall be
interpreted as non-limiting, i.e. meaning "consist at least
of".
[0206] The present invention is not limited to the above described
preferred embodiments. Various alternatives, modifications and
equivalents may be used. Therefore, the above embodiments should
not be taken as limiting the scope of the invention, which is
defined by the appending claims.
* * * * *